Combustible (flammable) gas detectors have been in use for many years for the prevention of explosive accidents. Conventional gas detectors operate by catalytic oxidation of combustible gases. Such gas detectors comprise a platinum wire coil encased in a refractory (for example, alumina) bead, the surface area of which is covered with a catalyst. This encased platinum coil is commonly referred to as a pelement or a pellister. A detailed discussion of pelement and catalytic combustible gas detectors comprising such pelement is found in Mosely, P. T. and Tofield, B. C., ed., Solid State Gas Sensors, Adams Hilger Press, Bristol, England (1987), the disclosure of which is incorporated herein by reference.
In general, the pelement operates as a miniature calorimeter used to measure the energy liberated upon oxidation of a combustible gas. The platinum element serves two purposes within the pelement: (1) heating the bead electrically to its operating temperature (typically approximately 500.degree. C. and (2) detecting changes in temperature produced by oxidation of the combustible gas.
The increase in temperature is measured in terms of the variation in resistance of the platinum element (with temperature variation) relative to a reference resistance. The two resistances are part of a Wheatstone bridge circuit. The voltage developed across the circuit when a combustible gas is present provides a measure of the concentration of the combustible gas. The reference resistor generally comprises a compensating, nonactive pelement matched as closely as possible with the pelement carrying the catalyst.
Typically, the active pelement and the compensating pelement are deployed within an explosion-proof housing and are separated from the surrounding environment by a porous metal frit. The porous metal frit allows ambient gases to pass therethrough but prevents the "flashback" of flames into the surrounding environment. An example of a modular gas detector cell incorporating such a frit is illustrated in FIG. 1A.
Combustible gas detectors may act in one of two modes: (1) diffusion of gas into the pelements and (2) forced flow of gas into the vicinity of the pelements. In the case of a gas detector operating in the diffusion mode, a gas detector cell (such as illustrated in FIG. 1A) is placed into an environment in which combustible gasses are to be detected. The gas detector cell is typically encased within the gas monitoring unit, not shown in FIG. 1A. The gases comprising the surrounding environment diffuse through the frit to contact the active and compensating pelements within the monitoring unit.
The amount of time required for environmental gases to diffuse to the frit, through the frit and to the pelements creates a delay in the response time of the monitoring unit. In certain uses, such a delay in determining the combustible gas content of the environment is undesirable. In such cases, environmental gases may be forced via pumping to the vicinity of the gas detector cell to reduce the response time.
The forced flow of gas to a detector housing is often required in remote sampling, for example, in which a probe is placed in fluid connection via tubing to the gas detector. An example of such a detector is the PASSPORT.RTM. detector available from Mine Safety Appliances Company of Pittsburgh, Pa. In that detector, environmental gases are pumped into a plenum located above a frit of a modular gas detector cell as illustrated in FIG. 1A. These gases then diffuse through the frit to contact the pelements.
Another example of a detector operating in a forced flow mode is the GASURVEYOR device available from GMI of Renfrew, Scotland. The gas detector cell used in that device is illustrated in FIG. 1B. As illustrated, environmental gases are pumped through the detector cell which is equipped with frits at the entrance and exit of the housing to prevent flashback. Environmental gases flow over and into two wells in which the active pelement and the compensating pelement are seated. In the case of the GASURVEYOR device, the flow rate is maintained sufficiently high to create turbulent flow through the detector cell such that a component of the flow enters the wells surrounding the pelements. Absent turbulent flow, environmental gases would diffuse into the wells surrounding the pelements from the forced flow (oriented perpendicular to such wells), but the response time would reflect the delay associated with such diffusion.
Stricter response time requirements for gas detectors continue to be imposed by consumers of gas detection systems. Therefore, manufacturers of gas detectors continue their attempt to minimize such response times while maintaining competitive pricing. It is thus very desirable to develop a "flow-through" or forced-flow gas detector with improved response time and low manufacturing costs.